The polymeric derivatives are polymers and copolymers, in which the side carboxylic groups have been replaced with non-hydrophilic groups, in particular with ester, thioester, nitrile, N-substituted amide or hydrazide, and anhydride groups. The decisive criterion by which these lenses differ from known hydrogel lenses is non-hydrophilicity, i.e., the inability to absorb physically more than 1% water when in constant contact with water or an aqueous medium. Non-hydrophilic plastics are either not wetted or only somewhat wetted with water and their surface is usually much harder than the surface of the cornea and other tissues of living organs, in particular the eye. They are mainly used in the manufacture of lenses because of their excellent optical properties, such as high transparency and refractive index of light, and also for their relatively good biological tolerance by living tissues and body liquids and their excellent stability in an organism. The ease with which such plastics can be processed to form lenses is also advantageous.
The most often used material for a lens is poly(methyl methacrylate) and, to a lesser extent, copolymers of methyl methacrylate with other esters of methacrylic acid or acrylic acid. The supporting parts, which serve for the proper fixation and centering of the lens, are sometimes made from other polymers, for example, from polypropylene, poly(vinyl ether), and others.
These lenses have, in addition to the above cited properties, some less advantageous features. It is, above all, the above-mentioned relatively high hardness, which may cause, under unfavorable conditions, irritation and pressure necroses. Further inconvenient properties are the tendency to create light reflexes, increased reaction on a foreign body, and the tendency to form non-transparent layers of cells, which may considerably deteriorate the excellent optical properties of the lenses after a long period of time.
Surface saponrfication of hydrogel lenses from lightly crosslinked poly(hydroxyethyl methacrylate) with alkaline hydroxides in the presence of a salt, the aqueous solutions of which do not swell the synthetic hydrogel, has been already proposed (Czechoslovak Patent Application 215-87). The presence of salts reduces the depth of the swelling gradient, which is undesirable. Also, hydrophilization is not of concern because the hydrogels are always hydrophilic.
The thickness of the greater swelling surface layer must not be too large, not only because of the abovementioned undesirable decrease in refractive index, but also because mechanical resistance and, above all, tear resistance (resistance against tearing and further propagation of a tear from the edge of the lens) strongly decrease with an increase in swelling capacity. The formation of light reflexes is insignificant in hydrogel swollen lenses because the difference in refractive indices of the swollen lens and the surrounding aqueous medium is small. The differences between the properties of known hard lenses, e.g., made from poly(methyl methacrylate), are so great that a simple transferring of knowledge about one material to another material is unthinkable because the effect of processing is not the same for both types.